The 'real' reason Tran couldn't get it is that Japan is FIRM in their 5 years continuous residency requirement and he was living and training in Canada. If he'd been willing to live and train in Japan, things might have been different. He was unwilling to do the basic first step to getting citizenship there- a step that is almost never (or possibly absolutely ever) waived. This is the same thing that is likely to stop him from getting US citizenship now.

You also have to learn Japanese, pass written and oral interviews/tests in Japanese, and renounce any other citizenship.

In all honesty, I don't think that citizenship there is really harder to get than in many other countries at least on paper. The difference is they are extremely strict in applying the rules and also that many people aren't willing to surrender another citizenship or go through the (very difficult and arduous for western language speakers) process of learning Japanese well enough for a written and oral test.

And they also don't allow dual citizenship, so he would have to give up his Canadian citizenship, which he had to consider as well.